1. Field of the Invention
The present invention relates to a method for producing a thermoplastic elastomer powder. More particularly, the present invention relates to a method for efficiently producing a thermoplastic elastomer powder having a sphere-reduced average particle diameter of 700 xcexcm or less, by an under water cut method.
2. Description of Related Art
Conventionally, sheet-like molded articles having complicated uneven patterns on the surface, such as leather grains, stitches or the like, are used as skin materials such as automobile interior parts and the like. As such, a molded article formed by powder-molding a polyvinyl chloride-based composition is used. However, the molded article has problems such as generating hydrogen chloride gas and the like in incineration of the molded article. Therefore, a substitute for the molded article is required. A proposed substitute are thermoplastic elastomer powders obtained by grinding, for example, a thermoplastic elastomer composition of an ethylene-xcex1-olefin-based copolymer rubber with a polyolefin-based resin by means of a mechanical grinding method such as a freeze grinding or the like (e.g. JP05001183A, JP05005050A).
However, such a thermoplastic elastomer powder is inferior in a falling property due to its irregular form in individual particle of the powder. When a molded article having a complicated form is produced, for example a molded article having a convex portion which is narrow and high, the molded article experiences failures at the edge of the convex portion, such as pinholes, underfill, and the like. Two steps are required for producing the thermoplastic elastomer powder: a thermoplastic elastomer composition is once molded into pellet, and the pellet is mechanically ground.
For solving such problems, there are known thermoplastic elastomer powders produced by methods such as under water cut methods and the like (e.g. JP10081793A) that have a specific melt property and powder property.
The under water cut method comprises discharging a thermoplastic elastomer in melt state through a plurality of orifices of a die into water, and cutting the discharged matter with a cutter blade rotating along the die surface to obtain powder of the thermoplastic elastomer.
Further, the powder is usually transported together with water, collected and dried with a centrifugal drier or the like.
When a molded article is produced by a powder molding method, such as a powder slush molding method or the like, using a thermoplastic elastomer powder having a specific melting property and powder property, it is necessary to suppress the sphere-reduced average particle diameter of the thermoplastic elastomer powder to 700 xcexcm or less, to control the thickness of a molded article to about 1.4 mm or less.
However, when a thermoplastic elastomer powder having a sphere-reduced average particle diameter of 700 xcexcm or less is used in the under water cut method, the die surface is exposed constantly to water and cooled, and the thermoplastic elastomer composition in melt state is solidified in the orifice causing clogging, and leading to a reduction in productivity.
Specifically, the particle diameter (Rxcex1) of a powder obtained by the under water cut method shows a relation of the following equation (1). When a powder having fine particle size is going to be produced, even if the average discharge amount (M) per orifice is reduced and the cutting speed (P) increases as production conditions, clogging rate (xcex1) increases leading to a problem of reduction in productivity.
Rxcex1={M/(xcfx80xc3x97Dxc3x97P)/(1xe2x88x92xcex1) xc3x971014}1/3xe2x80x83xe2x80x83(1)
M: average discharge amount per orifice (g/hourxc2x7orifice)
D: density of thermoplastic elastomer composition (Kg/m3)
P: cutting speed (the number of times/min.)
xcex1xc3x97100: clogging rate (%)
Rxcex1: particle size when clogging rate is xcex1 (xcexcm)
Herein, the average discharge amount per orifice is obtained by simply dividing the whole die discharge amount by the number of die orifices. The cutting speed is the number of cuttings of the thermoplastic elastomer extruded through die orifices per minute, and is a value obtained by multiplying the rotation number of a cutter by the number of blades attached to a cutter rotation part.
An object of the present invention is to provide a method for producing a thermoplastic elastomer powder having a sphere-reduced average particle diameter of 700 xcexcm or less by an under water cut method without lowering productivity.
Namely, the present invention relates to a method for producing powder of a thermoplastic elastomer having a sphere-reduced average particle diameter of 700 xcexcm or less by an under water cut method which comprises discharging the thermoplastic elastomer in melt state through a plurality of orifices of a die into water, and cutting the discharged matter with a cutter blade rotating along the die surface to obtain powder of the thermoplastic elastomer, wherein the method satisfies the following conditions (a) and (b):
(a): the temperature of the thermoplastic elastomer directly before the die is from 120 to 220xc2x0 C., and
(b): the temperature of the die is from 230 to 350xc2x0 C.